The present invention relates to an inspection device and a method for nondestructive inspection and evaluation of magnetically permeable elongated objects such as wire cables, rods, pipes, and the like. The invention is concerned more particularly with a magnetic inspection method and device for detecting structural faults in the objects.
Presently, wire cables, pipes and the like can be inspected for three types of defects. Each of the defects is described in detail below and is an important indicator of structural integrity of the elongated object.
First, the elongated object can be inspected for a distributed defect known as a loss of metallic cross-sectional area (LMA). The loss of metallic cross-sectional area in a cable or pipe may be due to corrosion, wear or abrasion.
Second, the elongated object can be inspected for a defect known as a localized fault (LF) such as broken wires within a cable or circumferential cracks along a wall of a gas pipe.
Third, the elongated object can be inspected for a defect known as a structural fault (SF). For example, structural faults in a pipe include longitudinal stress-corrosion cracks in the pipe wall, and hard spots. Likewise, in a wire cable structural faults include changes in wire contact patterns such as loose wires.
Magnetic inspection methods for detecting LMA and LF in elongated magnetically permeable objects are presently available. For example, my U.S. Pat. No. 4,659,991 uses a method to nondestructively, magnetically inspect an elongated magnetically permeable object for LMA and LF. The method induces a saturated magnetic flux through a section of the object between two opposite magnetic poles of a magnet means. The saturated magnetic flux within the object is directly related to the cross-sectional area of the magnetically permeable object. A magnetic flux sensing coil is positioned between the poles near the surface of the object and moves with the magnet means relative to the object in order to sense quantitatively the magnetic flux contained within the object.
Because of the direct relationship between the cross-sectional area and the magnetic flux contained within the magnetically saturated object, LMA is detected by connecting the magnetic flux sensing coil to an electronic integrator. Likewise, a magnetic leakage flux sensing coil is also positioned between the poles near the surface of the object and moves with the magnet means relative to the object in order to sense qualitatively magnetic leakage flux escaping the object because of a distortion in magnetic flux caused by a localized fault within the object. LF is detected by a differential magnetic leakage flux sensing coil.
Employing magnetic inspection devices for detecting distributed defects such as structural faults, however, has not been successful using the method described above. Structural defects, such as longitudinal stress-corrosion cracks in pipes or loose wires in wire ropes, usually do not produce sufficient magnetic leakage flux to be detectable by magnetic sensors.
The reason for the detection failure is that a structural fault does not necessarily entail a detectable loss in metallic cross-sectional area. For example, loose wire contact patterns in a wire cable are indicative of a loss in structural integrity but not necessarily indicative of a detectable loss in cross-sectional area. New methods are needed to overcome the problem of reliably detecting distributed structural faults in elongated objects.
One solution employs an eddy current technique as opposed to magnetic/leakage flux techniques. U.S. Pat. No. 4,827,215 addresses the problem of detecting structural faults in wire cable by a device employing a three pole magnet. The device has two end poles of the same polarity with an opposite polarity-shared common pole centered between the two end poles. The purpose of the three pole magnet is to induce two opposing longitudinal, saturated magnetic fields in adjacent sections of a wire rope, each magnetic field associated respectively with one of two adjacent sections of the wire cable and the common pole, and each of the two longitudinal fields extending to a different end pole. Adjacent the common pole, the wire cable experiences a change in direction of the longitudinal magnetic flux. A stagnation point is defined as the longitudinal location in the object where the longitudinal magnetic flux is zero as the flux changes in direction.
Because of Faraday's Law, eddy currents are generated in the wire cable near the common pole because of the change of the longitudinal magnetic flux experienced in the object as it moves relative to the magnetic inspection device. According to Lenz's Law, the eddy currents generate another magnetic flux that opposes the change of the longitudinal magnetic flux induced by the magnets. Because eddy currents are a function of the electromagnetic properties of the wire cable or pipe, any structural fault will correspondingly change the eddy current patterns. Change of the eddy current patterns in turn correspondingly changes the normally constant opposing magnetic flux generated by the eddy currents. This change of the opposing magnetic flux can be detected by magnetic sensors such as coils or Hall generators. Hence, a sensor located at the common pole can detect changes in the eddy currents owing to structural faults in the object. In addition, the location of a sensor at the stagnation point is conducive to measuring changes in eddy currents because of minimum interference from longitudinal magnetic flux induced by the magnets since the longitudinal magnetic flux is virtually zero at the common pole and the stagnation point.
The inspection device described in U.S. Pat. No. 4,827,215 uses an eddy current technique, but the existence of eddy currents in the context of magnetic inspection devices is known in the field. For example, a 1983 Swiss paper Die elektromagnetische Prufung von Drahtseilen by Urs Balthasar Meyer mentions eddy currents as parasites because they distort the longitudinal magnetic flux.
A problem with the above-mentioned magnetic inspection device is that the device uses a bulky three pole magnet. This prior art overlooks the fact that changing longitudinal magnetic flux for inducing eddy currents and a corresponding stagnation point for favorable sensor positioning is also present at an end pole. The existence of changing longitudinal magnetic flux and a corresponding stagnation point at an end pole allows the construction of a simpler magnetic inspection device for the detection of structural faults.
Accordingly, it is a general object of the present invention to provide a simpler method and apparatus for determining structural faults in a magnetically permeable elongated object. Structural faults such as longitudinal stress-corrosion cracks, hard spots, manufacturing flaws, residual stress or stress caused by bending or sagging in a pipe, and wire contact pattern changes in a wire cable caused by loose wires are more easily detected by the described method.
It is another object of the present invention to provide an eddy current-magnetic saturation method which significantly reduces background noise so as to detect structural faults with improved sensitivity.
In addition, it is a general object of the present invention to employ a method and apparatus that can be employed for detecting structural faults inside or outside an elongated object.